1. Field of the Invention
This invention relates to cooling systems for internal combustion engines and particularly to boiling liquid coolant systems for vehicular engines.
2. Description of the Prior Art
Conventional automotive cooling systems are pressurized, forced circulation, liquid systems in which water or an aqueous antifreeze mixture is circulated by an engine-driven pump in a single closed loop circuit between the engine water jacket, where heat is transferred to the liquid coolant from the cylinders, and a radiator where the heat absorbed by the coolant in the engine is transferred to air flowing through the radiator. A pressure relief valve in the radiator fill cap is set at a pressure high enough (typically 15 psig) to prevent boiling of the liquid coolant under the normal range of engine operating conditions.
To reduce engine warm up time, a thermostatic valve is positioned at the outlet of the water jacket. The valve opens only when the coolant temperature exceeds a predetermined value (e.g. about 90.degree. C. or higher). At coolant temperatures below the set point, no coolant can flow to or from the engine, so that the temperature of the relatively small proportion of the total system coolant that is trapped in the jacket will rise rapidly.
Although conventional pressurized single-phase liquid coolant systems are reliable and almost maintenance-free, they have several inherent drawbacks. Surface heat transfer coefficients for a fluid in the liquid phase are relatively low and vary with flow velocity. In the typical automotive cooling system, cooled liquid from the radiator enters the engine at the lower front part of the block, and heated liquid leaves from the top of the cylinder head. Consequently, the front cylinders will run cooler than the rear cylinders. In addition, it is not possible to obtain uniform velocity in the complex flow passageways inside the cooling jacket, so local hot spots develop throughout the engine. Such hot spots are believed to contribute to the production of oxides of nitrogen (NO.sub.x) in the exhaust gases.
Since the highest temperatures are generated in the combustion chambers at the top of the cylinders, and since the coolant flow is generally upward through the engine, the upper part of each cylinder wall is much hotter than the lower part. This temperature differential from top to bottom of the cylinder wall (some 20.degree. to 30.degree. C.) causes thermal distortion of the engine block and cylinder head, with consequent increased blow-by and oil consumption. An even greater problem is that wall quenching, which produces an unburned layer of gases on the relatively cool lower cylinder walls, is the source of excessive carbon monoxide (C0) and unburned hydrocarbons (HC) in the exhaust gases. It also results in poor fuel efficiency.
The desirability of using a two phase boiling liquid cooling system to reduce the temperature differential from bottom to top of the cylinders occurring in the conventional single phase liquid system has long been recognized, and numerous proposals have been made for boiling liquid cooling systems for both stationary and mobile internal combustion engines. Representative examples of automotive boiling liquid cooling systems include U.S. Pat. Nos. 1,632,583; 3,223,075; 3,312,204; 3,384,304; 2,649,082; 1,754,300; 1,323,366; 1,812,899; 1,838,450; 2,766,740; 2,825;317; 2,804,860; 2,926,641; 1,687,679; 2,681,643; 1,860,258; 2,403,218; 1,895,509; 1,630,068; 1,630,069; 1,630,070; 1,658,933; 1,658,934; 1,703,164; 3,168,080; 3,082,753; and 3,524,499. See also, "Dual-Circuit Ebullition Cooling for Automotive Engines", a paper presented at a meeting of the Society of Automotive Engineers, San Fransisco, Calif., Aug. 17-20, 1964, by A. A. Tacchella, J. A. Fawcett and A. N. Anderson; "Evaporative Cooling", by H. C. Harrison, The Journal of the Society of Automotive Engineers, Vol. XVIII, No. 2 (February 1926); and "Dow Chemical Fills Cooling Gap", Automotive Industries, Aug. 15, 1970, pp. 53-54.
In typical boiling cooling systems, liquid coolant is boiled within the cooling jacket of the engine, the vaporized coolant being withdrawn from the upper part of the cooling jacket and flowing to an air cooled radiator or condenser, either directly or through a separator tank. The condensate collects in a sump connected to the bottom of the condenser and is returned to the inlet to the cooling jacket or to a supply tank for gravity flow to the engine.
Since boiling occurs at constant temperature (assuming the pressure is held constant) and since surface heat transfer coefficients for fluids in the vapor state are much higher than for the same fluids in the liquid state, boiling cooling systems can maintain the cylinder wall temperatures more nearly constant from top to bottom, and the entire cylinder wall will be hotter, thereby reducing the production of CO and HC in the exhaust gases and improving fuel economy.
The potential benefits of boiling liquid cooling for automotive engines are, however, difficult to achieve in a practical system. A major problem with prior automotive boiling liquid cooling systems has been the need to constantly monitor the coolant supply and to frequently replenish coolant lost through the system vent. This is not merely an inconvenience; coolant loss may be so rapid that major damage can result before the engine can be shut down.
Heretofore, it has been impossible to eliminate coolant loss from such cooling systems due to vapor loss through the system vent under all engine operating conditions. For example, it has not been possible to eliminate vapor loss during high ambient air temperature engine operating conditions which result from either a low volume of air flowing across the condenser caused, e.g., by low engine speed during idling and hill climbing or when the vehicle is moving slowly, or which is due to engine shutdown, or when outside air temperatures are very high, e.g., on a hot summer day, under which conditions the capacity of the condenser of the cooling system to condense the vaporized coolant is exceeded by the rate at which vaporized coolant is generated in the system. It has also not been possible to eliminate vapor loss under engine operating conditions during which no liquid coolant fills the return line from the sump to the separator tank, such as, for example, during engine start-up when noncondensible gases are purged from the system and coolant vapor is generated by the heat of the engine but little coolant is condensed by the condenser, during engine shutdown when high ambient air temperatures increase the amount of coolant vapor generated but the amount of condensate produced by the condenser is reduced, and during low power engine operating conditions such as, e.g., coasting down a hill when little power is utilized and the condenser also produces little coolant condensate.
Another common problem with prior vapor cooling systems is the tendency of such systems to build up excessive pressure levels as the engine load increases. A typical prior art vapor system will operate at a pressure level of 15-25 p.s.i. at moderate engine loads and at a pressure level of 25-45 p.s.i. at high engine loads if not vented. As a result, most prior art vapor cooling systems utilize a pressure relief valve of one form or another to release the excess pressure from the system. Such relief valves, however, cause a constant loss of vapor and, as a result, a continuing reduction of the amount of coolant in the system which can range from moderate to severe, depending upon the location of the relief valve. It is therefore evident that such prior art vapor cooling systems are not low pressure systems but rather are high pressure systems which use a relief valve, and do not prevent coolant loss.
A true low-pressure vapor cooling system is extremely advantageous. At sea level, a rise of approximately two degrees F. in boiling point is caused by every one pound of pressure developed by the cooling system. Therefore, if a cooling system operates under a fluctuating pressure load of, for example, 15-25 p.s.i., the resulting variation of the engine temperature will range from 30.degree. to 50.degree. F. in excess of its optimum operating temperature. The quick release effect of a pressure relief valve not only causes a coolant loss from the system as a result of its operation, it only partially aids in reducing the adverse effects of the vapor cooling system operating at a high pressure. The rating of a 2 p.s.i. pressure relief valve, for example, is really only an average 2 p.s.i. value since the pressure rises and falls above and below that point as the pressure at the valve builds, and then the valve releases and relieves the pressure. The shock effect of this type of relief in a vapor cooling system causes constant flashing of vapor in critical areas of the engine, particularly around the exhaust valve jacketing, the combustion chamber dome, and the exhaust port areas of the engine.
A major problem in the prior art vapor cooling systems which has gone unrecognized and causes such pressure build up is the phenomenon of vapor build-up or "back pressure" from the condenser of such systems which causes a pressure rise in the engine cooling jacket, although not in the condenser. If such a pressure rise is not checked, the result would be a vapor pocket formed at the uppermost part of the cooling jacket which as it expands in size would displace liquid coolant from the coolant jackets in the engine, eventually causing the cooling system to fail. The tubes of a condenser must, of course, be long enough so that vapor is condensed by the time it reaches the ends of the condenser tubes. However, as the length of the tubes is increased, the condenser becomes restrictive to vapor flow, which causes a back-pressure to develop as the engine produces more and more vapor, which in turn causes the formation of "hot vapor pockets" in the coolant located in the engine head and a resulting rise in the boiling temperature of the liquid coolant due to increased pressure and the engine operating temperature. This is particularly important because not only has vapor pocketed but in so doing, it has precluded liquid coolant from that space and no heat is transferred from the engine cooling jacket walls. Since under these conditions all of the vapor generated cannot flow into the condenser to be condensed into liquid, a back-up of vapor occurs at the entrance to the condenser and a continuing rise in pressure in the system also results which continues until the engine load is decreased and the volume of vapor is reduced. The condenser is, thus, a restrictive orifice and only permits the flow of a limited volume of vapor. Once the volume of vapor generated exceeds this maximum, pressure builds as vapor "backs up". The use of a pressure relief valve in prior art vapor cooling systems is not sufficient to overcome this problem and causes vapor loss.
Heretofore, the condensers in vapor cooling systems have been utilized only as heat exchangers, and not as a means for controlling the pressure level of a vapor cooling system. Various types of condensers for use in automotive cooling systems, including boiling liquid cooling systems, are disclosed in U.S. Pat. Nos. 1,329,419, 1,376,086, 1,432,518, 1,558,009, 1,658,090, 1,700,270, 1,702,910, 1,706,693, 1,767,598, 1,806,382 and 3,223,075.
As a result of the foregoing problems, prior art boiling liquid cooling systems have not been commercially utilized.